Iv tested 2 of the filaments so far with 2.5v ac and looks like there ok,there seems to be mercury in them still! how would i test for emission without ionizing? Is ionizing what it does when it turns bule inside when voltage is applied to the anode? I dont have any good way of testing them, was wondering whats the minimum voltage i can try on them? and how could i do it?

Assuming you have no tube tester that will check them, here's a quick and dirty way of doing so. You'll need a 2½ volt transformer to power the filament. And you'll need a metered 100 volt AC (or DC) supply and a 100 watt wire wound resistor of at least 100 or 200 ohms. Light the filament and connect the tube across the 100 volt supply with the resistor in series. For 100 ohms you should draw almost 1 amp from the supply. With 200 ohms you'll draw ½ amp and so on. You could even use lower or higher voltage depending on what you have and/or feel comfortable with. But you'll want to draw at least ¼ amp through the tube for a meaningful test.

A word of caution. If you have modern day 866's with the smaller envelope, there's not much to worry about. But if you have new old stock from years back with the larger (fatter) glass envelope, these tubes have a problem. It seems that the coating on the plate as well as the cathode shield tend to flake and peel off. When this happens, the flakes can get between the plate element and the cathode and short the tube and your supply out. I've seen this happen many times with the older stock 866's. Even old 816's and 872's have this flaking problem. This may be hard to see at first because as the mercury vaporizes it clouds the inside of the tube. So you have to pay close attention if you have older stock. And even if the flakes go to the bottom of the inside, there's no guarantee they'll stay there. I would not use one in this condition.

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"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers. That is a fairy story for people afraid of the dark." ~Stephen Hawking

Another comment - don't lay the tube on its side while testing. They should be upright.

If the tube shorts as HollowState cautions, you have no problem because the resistor will limit the current. It depends on whether you are using an ac or dc supply, just how much current you will measure. Since it only conducts on half the cycle of ac you will show a lower reading. Incidentally, a 100 Watt light bulb might be a good, cheap limiting resistor.

Yes, what Bob said above. Another test is to measure the voltage drop across the tube. It shouldn't be much more then 15 volts for an 866.

__________________
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers. That is a fairy story for people afraid of the dark." ~Stephen Hawking

You can use light bulbs (the common incandescent domestic types of various wattages) in place of power resistor, they will work just as well on DC. Light bulbs are rated for tens or hundreds of watts of dissipation and don't cost a fortune in case they die.

Note that light bulb's resistance is considerably lower when cold, but it goes up as it warms up (PTC).

You should start with low wattage types first (these draw less current) and move on to higher wattages if everything works as expected.

__________________modverb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."

Using an incandescent lamp as the series resistor, while it will function to a degree, will only give you empirical results that will have to be interpreted. Since the resistive element is auto-variable (spongy), current through the tube will vary as will as the voltage across it. A resistor of fixed known value will provide a much more discernable answer. So in other words, for a meaningful test setup there should only be one variable element, the tube itself.

__________________
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers. That is a fairy story for people afraid of the dark." ~Stephen Hawking

Using an incandescent lamp as the series resistor, while it will function to a degree, will only give you empirical results that will have to be interpreted. Since the resistive element is auto-variable (spongy), current through the tube will vary as will as the voltage across it. A resistor of fixed known value will provide a much more discernable answer. So in other words, for a meaningful test setup there should only be one variable element, the tube itself.

I agree with Hollowstate - you absolutely do not want more than one variable in the equation if you are attempting to verify these tubes are good.

Also do not exceed the continuous maximum rated current as you test them.

I've had a few of those older NOS tubes with the flaking plates, sadly I'd pitch any encountered.